The present invention relates to a liquid crystal display, and more particularly to a liquid crystal display using ferroelectric or anti-ferroelectric liquid crystal with an improved visible angle dependency.
The liquid crystal display has widely been used in various fields for watches, pocket calculators, word processors and personal computers. The liquid crystal phase used in the liquid crystal display is normally nematic phase. The liquid crystal display using the nematic liquid crystal has a serious problem in visible angle dependency, wherein display may change in color over angles of view since the nematic liquid crystal varies in transmittance of light over view angles. The range of view angle free of any change in color is relatively narrow
The following description will focus on the reason why the nematic liquid crystal varies in transmittance of light over view angles. The liquid crystal molecule has a slender shape having a longitudinal axis. The transmittance of light through the liquid crystal depends upon an included angle defined by the longitudinal axis of the slender-shaped liquid crystal molecule and a direction of a ray of light transmitting through the liquid crystal. For a nematic liquid crystal, it is convenient to direct the attention onto the longitudinal axis of the liquid crystal molecules in the center area of a display cell, excluding liquid crystal molecules adjacent to a pair of substrates sandwiching the twisted liquid crystal. If no voltage is applied, the liquid crystal molecules in the center area of the cell are oriented so that the longitudinal axis of the liquid crystal molecules is directed in parallel to the substrate surface. If, however, a voltage is applied, the liquid crystal molecules are moved in a plane vertical to the substrate surface so that the longitudinal axis of the liquid crystal molecules is risen up and tilted from the substrate surface as illustrated in FIG. 1. Since the direction of the ray of light depends upon a relative position of observer to the screen of the display, the included angle defined by both the longitudinal axis of the liquid crystal molecule and the direction of the ray of light is varied depending upon the change in the relative position of observer to the screen of the display. As described above, the transmittance of the light through the liquid crystal depends upon the included angle defined by both the longitudinal axis of the liquid crystal molecule and the direction of the ray of light. FIG. 1 is a schematic view illustrative of the twisted nematic liquid crystal molecule which is tilted and risen up from the substrate surface to explain the dependency upon the view angle of the twisted nematic liquid crystal molecules. For example, as illustrated in FIG. 1, the liquid crystal molecule 5 in the center area of the liquid cell 11 is risen so that the longitudinal direction thereof is tilted from the substrate surface. The ray of light 14a having been transmitted through the liquid crystal cell is largely different in direction from the longitudinal axis of the liquid crystal molecule, for which reason the included angle defined by the longitudinal axis of the liquid crystal molecule and the direction of the ray of light 14a is large. Since the transmittance of the ray of light having been transmitted through the liquid crystal depends upon the included angle defined by the longitudinal axis of the liquid crystal molecule and the direction of the ray of light 14a, the transmittance of the ray of light 14a through the liquid crystal is low. Accordingly, if the observer views the screen in the direction of an arrow mark 13a in parallel to the ray of light 14a, then the transmittance of the ray of light 14a through the liquid crystal is low. In contrast, the ray of light 14b having been transmitted through the liquid crystal cell 11 is almost the same in direction as the longitudinal axis of the liquid crystal molecule, for which reason the included angle defined by the longitudinal axis of the liquid crystal molecule and the direction of the ray of light 14a is small. Since the transmittance of the ray of light having been transmitted through the liquid crystal depends upon the included angle defined by the longitudinal axis of the liquid crystal molecule and the direction of the ray of light 14b, the transmittance of the ray of light 14b through the liquid crystal is high. Accordingly, if the observer views the screen in the direction of an arrow mark 13b in parallel to the ray of light 14b, then the transmittance of the ray of light 14b through the liquid crystal is high. As described above, the nematic or twisted nematic liquid crystal display has the above problem in a remarkable dependency upon the view angle.
In order to settle the above problem in the remarkable dependency upon the view angle, it was proposed to divide the orientation of the twisted nematic liquid crystal molecules into two different orientations over two divided areas for the purpose of reduction in dependency upon the view angle. This technique is disclosed in Japanese laid-open patent publication No. 63-106624. Two types of areas in different two orientation directions co-exit in each pixel. FIG. 2 is a schematic view illustrative of the twisted nematic liquid crystal molecules which are tilted and risen up in different two orientations from the substrate surface to explain the dependency upon the view angle of the twisted nematic liquid crystal molecules. Each pixel is divided into two types of the area differing in orientation by 180 degrees from each other wherein the two types of area co-exist in a local part of each pixel. The two types of the area differ in view angle dependency by 180 degrees from each other and co-exist locally in the each pixel so that the different dependencies of view angle may be canceled totally. If the observer views the screen of the display in a direction of an arrow mark 13a, then the rays of light 14a and 14c are taken into eyes of the observer. In this case, as well illustrated in FIG. 2, the ray of light 14a have been transmitted through the twisted nematic liquid crystal molecule 5 which is risen up toward the right-up direction and tilted from the substrate surface. Since the included angle defined by the transmission direction of the ray of light 14a and the longitudinal axis of the liquid crystal molecule is large, the transmittance of the ray of light 14a is low. In contrast, the ray of light 14c have been transmitted through the twisted nematic liquid crystal molecule 5 which is risen up toward the left-up direction and tilted from the substrate surface. Since the included angle defined by the transmission direction of the ray of light 14c and the longitudinal axis of the liquid crystal molecule is small, the transmittance of the ray of light 14c is high. Since the observer can view both the rays of light 14a and 14c having low and high transmittances, the dependency of the view angle is apparently reduced.
On the other hand, if the observer views the screen of the display in a different direction of an arrow mark 13b, then the rays of light 14b and 14d are taken into eyes of the observer. In this case, as well illustrated in FIG. 2, the ray of light 14b have been transmitted through the twisted nematic liquid crystal molecule 5 which is risen up toward the right-up direction and tilted from the substrate surface. Since the included angle defined by the transmission direction of the ray of light 14b and the longitudinal axis of the liquid crystal molecule is small, the transmittance of the ray of light 14b is high. In contrast, the ray of light 14d have been transmitted through the twisted nematic liquid crystal molecule 5 which is risen up toward the left-up direction and tilted from the substrate surface. Since the included angle defined by the transmission direction of the ray of light 14d and the longitudinal axis of the liquid crystal molecule is large, the transmittance of the ray of light 14d is low. Since the observer can view both the rays of light 14b and 14d having low and high transmittances, the dependency of the view angle is apparently reduced.
The dependency of view angle of the twisted nematic liquid crystal divided into co-existent different two types of area with reference to FIG. 3 which is a view illustrative of the definitions of the direction of observation 13, the polar angle .theta. and the azimuth angle .phi. from a liquid crystal cell 11. An evaluation point is set on the origin O on the liquid crystal cell 11 so that the observer observes the evaluation point on the origin O in the observation direction 13 for measurement of the transmittance of light. FIG. 4 is a diagram illustrative of variations in transmittance of light through the twisted nematic liquid crystal divided into co-existent different two types of area of FIG. 3 over variable incident angles when the polar angle is varied in the range of -70 degrees to +70 degrees with the azimuth angle fixed at 90 degrees. The twisted nematic liquid crystal display is gray-scaled. In the range of the polar angle from -40 degrees to +40 degrees, the gray scale is kept in the normal order. If the polar angle is beyond the range of -40 degrees to +40 degrees, the inversion of the gray scale is observed. A distance between adjacent curves of the transmittances is not constant. The compensation to the dependency of view angle by dividing the twisted nematic liquid crystal into the co-existent different two types of area is effective but only within the range of the polar angle from -40 degrees to +40 degrees. If the polar angle is beyond the range of the polar angle from -40 degrees to +40 degrees, this compensation is ineffective. As described above, the twisted nematic liquid crystal has the above problem in narrow visible angle.
In place of such twisted nematic liquid crystal, the ferroelectric liquid crystal having smectic C* phase is attractive due to its relatively wide visible angle. For example, a surface stabilized ferroelectric liquid crystal mode is disclosed as one of the ferroelectric liquid crystal and reported by N. A. Clark and S. T. Lagerawll in Applied Physics Letter Vol. 36 (1989). The ferroelectric liquid crystal is provided in a narrow cell gap so that the ferroelectric liquid crystal has the helical free structure wherein liquid crystal director is in the bistable states depending upon application of a voltage.
Alternatively, a unistable mode liquid crystal is disclosed in Japanese laid-open patent application No. 4-212126. Further alternatively, a deformed helix ferroelectric mode liquid crystal is disclosed in Advances in Liquid Crystal Research and Applications, 1980 p. 469. Further more, anti-ferroelectric liquid crystal is disclosed in Ferro-Electronics Vol. 149, pp. 255.
Those ferroelectric and anti-ferroelectric liquid crystal displays have wide visible angles for the following reasons. Normally, the ferroelectric or anti-ferroelectric liquid crystal is injected into a cell having been treated with parallel or anti-parallel orientation so as to order the longitudinal axis of the liquid crystal molecules. It is also disclosed in Japanese laid-open patent publication No. 4-371925 that, in order to order the longitudinal axis of the liquid crystal molecules, the rubbing direction of an upper substrate crosses to the rubbing direction of a bottom substrate. For those reasons, it is possible to deal with the ferroelectric and anti-ferroelectric liquid crystal as an uniaxial double refraction liquid crystal. In this model, the liquid crystal molecules lie so that the longitudinal axis of the liquid crystal molecules is parallel to the substrate surface. Upon application of the voltage or no application of the voltage, the liquid crystal molecules shows such a motion that the ends of the longitudinal axis thereof rotates around the normal of the substrate surface and in a plane parallel to the substrate surface so that the locus of the longitudinal axis on rotation draws two cones which tops faces to each other. FIG. 5 is a schematic view illustrative of the ferroelectric or anti-ferroelectric liquid crystal molecules showing a rotation motion such the locus of the longitudinal axis on rotation draws two cones which tops faces to each other. Similarly to the twisted nematic liquid crystals, the transmittance of light through the ferroelectric or anti-ferroelectric liquid crystal depends upon the included angle defined by both the longitudinal axis of the ferroelectric or anti-ferroelectric liquid crystal molecules and the direction of the ray of light having been transmitted through the ferroelectric or anti-ferroelectric liquid crystal. In FIG. 5, two rays of light 14a and 14b have been transmitted through a ferroelectric or anti-ferroelectric liquid crystal molecule 5. If the observer views the display screen in a direction 13a, then the observer observes the ray of light 14a. If, however, the observer views the display screen in a direction 13b, then the observer observes the ray of light 14b. The included angle defined between the direction of the ray of light 14a and the longitudinal axis of the ferroelectric or anti-ferroelectric liquid crystal molecule 5 is equal to the included angle defined between the direction of the ray of light 14b and the longitudinal axis of the ferroelectric or anti-ferroelectric liquid crystal molecule 5, for which reason the transmitance of the ray of light 14a is the same as that of the ray of light 14b. The observer can observe the same quality of photon or the intensity of light both in the directions 13a and 13b. This means that if the viewer direction is tilted and varied, then the transmittance of the ray of light through the ferroelectric or anti-ferroelectric liquid crystal molecule 5 is symmetrically varied. This can been said when the display is gray-scaled. Namely, even if the display is gray-scaled, then the transmittance of the ray of light through the ferroelectric or anti-ferroelectric liquid crystal molecule 5 is symmetrically varied. FIG. 6 is a diagram illustrative of variations in transmittance of the ray of light versus incident angle or view angle in causes of applications of various voltages V0&lt;V01&lt;V2&lt;V3 for realizing four gray-scale display, wherein polar angle of the view direction is varied in the range of -70 degrees to +70 degrees toward directions having the azimuth angles of 0 degree and 180 degrees. The transmittances of the ray of light are varied symmetrically with reference to the zero polar angle of the view direction. No inversion on gray scale appears not only in the small polar angle range but also near the polar angle of +70 degrees when the voltages V0, V1 and V3 are applied.
In case of application of the voltage V2, there still remains the problem in dependency upon view angle. Namely, the transmittance of the ray of light increases as the incident angle approaches .+-.50 degrees. As the incident angle or the view angle approaches zero, the transmittance of the ray of light decreases and is lower by near 10% from when the incident angle approaches .+-.50 degrees. The ferroelectric or anti-ferroelectric liquid crystal display shows symmetrical variations in transmittance of the ray of light over incident angles for all of the four gray-scales corresponding to V0, V1, V2 and V3. Notwithstanding, in the intermediate gray scale corresponding to V2, the transmittance of the ray of light is lower in the front view or at the zero polar angle and as the view direction is tilted from the front view, the transmittance of the ray of light increases.
The above described ferroelectric or anti-ferroelectric liquid crystal display has another problem in flicker in driving the display. Generally, the liquid crystal display is driven by alternating current driving wherein periodical applications of positive and subsequent negative voltages are made in order to avoid destruction of the liquid crystal due to a direct current component. The ferroelectric or anti-ferroelectric liquid crystal display utilizes the interaction between electric field and spontaneous polarization, for which reason the ferroelectric or anti-ferroelectric liquid crystal changes in orientation in accordance with the polarity of the applied voltage. In the light of optical responsibility, the change in orientation of the ferroelectric or anti-ferroelectric liquid crystal molecules corresponds to that the longitudinal axis of the ferroelectric or anti-ferroelectric liquid crystal rotates around the normal of the substrate surface and the ends of the ferroelectric or anti-ferroelectric liquid crystal rotate in a plane parallel to the substrate surface so that the locus of the longitudinal axis on rotation draws two cones which tops faces to each other.
If the observer views the display screen in the front direction, then the observer views the ray of light having been transmitted through the ferroelectric or anti-ferroelectric liquid crystal molecules in a direction parallel to the normal of the substrate surface, for which reason the included angle defined between the ray of light and the longitudinal axis of the ferroelectric or anti-ferroelectric liquid crystal molecules remains unchanged when the longitudinal axis of the ferroelectric or anti-ferroelectric liquid crystal rotates around the normal of the substrate surface and the ends of the ferroelectric or anti-ferroelectric liquid crystal rotate in a plane parallel to the substrate surface so that the locus of the longitudinal axis on rotation draws two cones which tops faces to each other. Since the transmittance of the ray of light through the ferroelectric or anti-ferroelectric liquid crystal depends upon the included angle defined between the ray of light and the longitudinal axis of the ferroelectric or anti-ferroelectric liquid crystal molecules, the transmittance remains unchanged during the above rotation motion of the ferroelectric or anti-ferroelectric liquid crystal molecules.
If, however, the observer views the display screen in the oblique direction, then the observer views the ray of light having been transmitted through the ferroelectric or anti-ferroelectric liquid crystal molecules in a direction tilted from the normal of the substrate surface, for which reason the included angle defined between the ray of light and the longitudinal axis of the ferroelectric or anti-ferroelectric liquid crystal molecules is changed when the longitudinal axis of the ferroelectric or anti-ferroelectric liquid crystal rotates around the normal of the substrate surface and the ends of the ferroelectric or anti-ferroelectric liquid crystal rotate in a plane parallel to the substrate surface so that the locus of the longitudinal axis on rotation draws two cones which tops faces to each other. Since the transmittance of the ray of light through the ferroelectric or anti-ferroelectric liquid crystal depends upon the included angle defied between the ray of light and the longitudinal axis of the ferroelectric or anti-ferroelectric liquid crystal molecules, the transmittance is changed during the above rotation motion of the ferroelectric or anti-ferroelectric liquid crystal molecules. Actually, if the frequency of the applied voltage is about 60 Hz, then the phenomenon of flicker appears. The above problem with the flicker prevents realization of the fill color wide view angle liquid crystal display.
In the above circumstances, it is required to develop a novel ferroelectric or anti-ferroelectric liquid crystal display within an improved dependency of view angle and being free from flicker in alternating current driving the display.